Agents sent in a felon to buy from Strunk as part of an undercover sting. They arrested Strunk soon thereafter on charges of selling a weapon to a felon and selling without a license.

Appearing before U.S. District Judge Orlando Garcia in February, when he received a 10-year-sentence, Strunk, 59, an Army veteran, insisted he did nothing illegal and claimed "the law is too broad.''

In the aftermath of the Newtown, Conn., massacre, the focus on gun shows mostly concerns the effort by gun-control advocates to close the "gun show loophole'' - the fact that unlike sales by federally licensed dealers, private transactions at shows and elsewhere do not require background checks. Last week, the Senate Judiciary Committee on a party-line vote approved a universal background check measure by Sen. Charles Schumer, D-N.Y.

But what stands out about the Strunk case is not the background-check issue but rather that the ATF - the gun lobby's perennial whipping boy - pursued it at all, given the political firestorm that erupted on Capitol Hill over previous gun show investigations.

ATF got its knuckles rapped by a Republican-dominated House Judiciary subcommittee in 2006 when a gun show promoter, two gun sellers and a private investigator hired by the National Rifle Association accused agents of unduly harassing patrons at gun shows in Richmond, Va.

A 2007 Justice DepartmentInspector General's report exonerated ATF and concluded that the agency's 202 gun show-related operations nationwide over the previous three years were based on "intelligence from a variety of sources indicating that illegal activity was occurring or was about to occur at a specific gun show."

The 202 investigations represented 3.3 percent of the estimated 6,000 gun shows held during this period, the IG report concluded. The report found that 11 of the operations were conducted by ATF's Houston division (which also has field offices in San Antonio, Beaumont and Laredo). Several of those were in the Rio Grande Valley, targeting gun-show purchases intended for Mexican drug cartels across the border.

ATF issued a "reminder'' to agents on gun show etiquette early in 2006, but agents still had a green light to investigate suspects involved in gun shows.

Even so, a subtle message ricocheted through ATF to avoid gun show-related investigations of non-specific illegal activity, according to two former agents.

"It was never formalized, but any time one of your peers is dragged before Congress, and there's an inspector general report, you get the feeling that it would just hurt your career,'' said David Chipman, who retired last year after 25 years in ATF and now serves as a consultant for Mayors Against Illegal Guns. "The lesson is there's no big reward if it goes well compared to the downside risk."

Although the vast majority of gun shows attract ordinary firearms enthusiasts who enjoy exchanges with those who share a common interest, they also are junctures for guns that end up in criminal hands.

Thirty percent of guns involved in federal firearms trafficking investigations are connected to gun shows, according to ATF data cited in a 2009 Mayors Against Illegal Guns report.

An ATF spokeswoman said the agency pursues all leads, even those that point to gun shows.

"We always try to stay ahead of trafficking schemes while respecting the individual rights of citizens to bear arms,'' said the spokeswoman, Ginger Colbrun.

ATF now requires all its field offices to man informational booths at two or more gun shows a year, Colbrun said. The aim is to educate licensed firearms dealers, private sellers and patrons on federal gun statutes and licensing requirements, she said.

Two prosecutions

In San Antonio since 2008, only two gun show-related ATF investigations have resulted in prosecutions.

In addition to Strunk, agents arrested Celerino "Cele" Castillo III of Pharr in 2008 for using a straw purchaser he met at a gun show in San Antonio to buy firearms that he resold.

Of 32 guns he obtained and resold, 23 were FN Five-seveN pistols known in Mexico as "matapolicias" or "cop killers," because they can fire armor-piercing ammo. Prosecutors alleged Castillo provided them to others who smuggled the weapons to Mexico's cartels.

In an interview, Castillo disputed most of the accusations.

"I made a major mistake, and I have great remorse for this,'' Castillo said. "I did it to supplement my income, and … I am paying the price for it.''

He said he simply followed the practice of other sellers at gun shows he attended.